1. Field of the Invention
The present invention relates to a switch having the function of interrupting a light path under the effect of magnetic control.
2. Prior Art
Electricity and electronics are widespread Switches and keyboards have become very numerous in conjunction therewith. Being sensitive to wear, bounce, water, static electricity, and sparking, they are not without operational defects. A magnetically controlled optical switch can avoid them. Enclosed in a sealed box it is insensitive to liquids and to air-borne dirt.
A first implementation of this type of switch is described in French patent application FR-A-2 595 022. It consists in placing a metal ball in a cylindrical hole passing through the middle of the light path. When a magnet is moved close to the ball, the ball rises and either interrupts or else disengages the light path. However, this system suffers from the defect of being sensitive to shock and vibration
A second embodiment, eliminating such sensitivity to vibration, is described in the above-mentioned patent application It consists in taking a relatively fine metal blade to constitute a spring which is sensitive to magnetic attraction, and in folding one of the ends of the blade in order to form a shutter for interrupting the light path, with the other end of the blade being fixed to the base of the switch.
As described, this switch does not operate reliably. In an office keyboard having about 100 keys, it has proved practically impossible to ensure that all of the keys are in working condition simultaneously.
In use, the switch includes certain limitations which give rise to friction in the operation of the switch and to the danger of jamming. As described, the shutter tends to describe an arc of a circle in its housing. This means that a wide shutter slot needs to be provided in the switch, and this is detrimental when optical fibers are used to constitute the light path, since the wider the slot, the greater the loss of light. If the slot is made narrower, it is observed that the shutter tends to rub against the edges of one of the fibers and to jam in the switch base.
Document EP-Al-0 115 126 (Frequency Control Products Inc.) describes a switch for single optical fibers. As shown in its FIGS. 5 and 6, this switch comprises a base with a light path going therethrough, together with a guide slot intersecting the path. A larger volume is provided to one side of the guide slot and contains a shaft carrying a pivoting disk. A thin blade extends from the disk into the slot, and depending on the angular position of the disk, the blade either interrupts or does not interrupt the light path.
A magnet is disposed on the other side of the disk and is actuated by a second, moving magnet placed outside the base. A spring returns the blade towards a rest position and an abutment limits the displacement due to the spring. This structure comprises at least five moving parts inside the base, it is complex and it is not suitable for an application which requires a large number of cheap switches, e.g. an office keyboard which generally comprises about 100 keys. In addition, the switch is bulky.
In contrast, the use of an inside magnet and of an outside magnet ensures that the force transmitted is adequate for reliable operation.